security: Fix critical vulnerabilities and harden application

## Security Fixes

### CRITICAL: Command Injection (adminBackup.js)
- Replaced exec() with spawn() using argument arrays
- Added input sanitization for host, user, and ssh_key
- Added regex validation for hostname/IP format
- Added username format validation
- Added SSH key file existence check
- Prevents shell metacharacter injection attacks

### HIGH: Hardcoded Password (set-admin-password.js)
- Removed hardcoded 'admin123' password
- Now requires password as CLI argument or env variable
- Added password strength validation (8+ chars, mixed case, numbers, special chars)
- Added --help flag with usage instructions
- Invalidates existing sessions on password change

### MEDIUM: XSS Vulnerability (WelcomeMessageEditor.tsx)
- Added DOMPurify sanitization to getPreviewHtml()
- Strips all HTML tags before rendering preview
- Prevents script injection in admin preview

### LOW: Sample Password Exposure (EmailConfigPage.tsx)
- Replaced plaintext sample password with masked placeholder
- Uses '••••••••' instead of realistic password

## Dependency Updates
- Fixed npm audit vulnerabilities (jws, qs, express)
- Backend: 0 vulnerabilities
- Frontend: 0 vulnerabilities

backend/package-lock.json

@@ -4355,21 +4355,6 @@
         "node": ">= 0.8"
       }
     },
-    "node_modules/body-parser/node_modules/qs": {
-      "version": "6.14.0",
-      "resolved": "https://registry.npmjs.org/qs/-/qs-6.14.0.tgz",
-      "integrity": "sha512-YWWTjgABSKcvs/nWBi9PycY/JiPJqOD4JA6o9Sej2AtvSGarXxKC3OQSk4pAarbdQlKAh5D4FCQkJNkW+GAn3w==",
-      "license": "BSD-3-Clause",
-      "dependencies": {
-        "side-channel": "^1.1.0"
-      },
-      "engines": {
-        "node": ">=0.6"
-      },
-      "funding": {
-        "url": "https://github.com/sponsors/ljharb"
-      }
-    },
     "node_modules/body-parser/node_modules/type-is": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/type-is/-/type-is-2.0.1.tgz",
@@ -5766,39 +5751,39 @@
       }
     },
     "node_modules/express": {
-      "version": "4.21.2",
-      "resolved": "https://registry.npmjs.org/express/-/express-4.21.2.tgz",
-      "integrity": "sha512-28HqgMZAmih1Czt9ny7qr6ek2qddF4FclbMzwhCREB6OFfH+rXAnuNCwo1/wFvrtbgsQDb4kSbX9de9lFbrXnA==",
+      "version": "4.22.1",
+      "resolved": "https://registry.npmjs.org/express/-/express-4.22.1.tgz",
+      "integrity": "sha512-F2X8g9P1X7uCPZMA3MVf9wcTqlyNp7IhH5qPCI0izhaOIYXaW9L535tGA3qmjRzpH+bZczqq7hVKxTR4NWnu+g==",
       "license": "MIT",
       "dependencies": {
         "accepts": "~1.3.8",
         "array-flatten": "1.1.1",
-        "body-parser": "1.20.3",
-        "content-disposition": "0.5.4",
+        "body-parser": "~1.20.3",
+        "content-disposition": "~0.5.4",
         "content-type": "~1.0.4",
-        "cookie": "0.7.1",
-        "cookie-signature": "1.0.6",
+        "cookie": "~0.7.1",
+        "cookie-signature": "~1.0.6",
         "debug": "2.6.9",
         "depd": "2.0.0",
         "encodeurl": "~2.0.0",
         "escape-html": "~1.0.3",
         "etag": "~1.8.1",
-        "finalhandler": "1.3.1",
-        "fresh": "0.5.2",
-        "http-errors": "2.0.0",
+        "finalhandler": "~1.3.1",
+        "fresh": "~0.5.2",
+        "http-errors": "~2.0.0",
         "merge-descriptors": "1.0.3",
         "methods": "~1.1.2",
-        "on-finished": "2.4.1",
+        "on-finished": "~2.4.1",
         "parseurl": "~1.3.3",
-        "path-to-regexp": "0.1.12",
+        "path-to-regexp": "~0.1.12",
         "proxy-addr": "~2.0.7",
-        "qs": "6.13.0",
+        "qs": "~6.14.0",
         "range-parser": "~1.2.1",
         "safe-buffer": "5.2.1",
-        "send": "0.19.0",
-        "serve-static": "1.16.2",
+        "send": "~0.19.0",
+        "serve-static": "~1.16.2",
         "setprototypeof": "1.2.0",
-        "statuses": "2.0.1",
+        "statuses": "~2.0.1",
         "type-is": "~1.6.18",
         "utils-merge": "1.0.1",
         "vary": "~1.1.2"
@@ -5836,15 +5821,6 @@
         "node": ">= 8.0.0"
       }
     },
-    "node_modules/express/node_modules/cookie": {
-      "version": "0.7.1",
-      "resolved": "https://registry.npmjs.org/cookie/-/cookie-0.7.1.tgz",
-      "integrity": "sha512-6DnInpx7SJ2AK3+CTUE/ZM0vWTUboZCegxhC2xiIydHR9jNuTAASBrfEpHhiGOZw/nX51bHt6YQl8jsGo4y/0w==",
-      "license": "MIT",
-      "engines": {
-        "node": ">= 0.6"
-      }
-    },
     "node_modules/express/node_modules/debug": {
       "version": "2.6.9",
       "resolved": "https://registry.npmjs.org/debug/-/debug-2.6.9.tgz",
@@ -7837,12 +7813,12 @@
       }
     },
     "node_modules/jws": {
-      "version": "3.2.2",
-      "resolved": "https://registry.npmjs.org/jws/-/jws-3.2.2.tgz",
-      "integrity": "sha512-YHlZCB6lMTllWDtSPHz/ZXTsi8S00usEV6v1tjq8tOUZzw7DpSDWVXjXDre6ed1w/pd495ODpHZYSdkRTsa0HA==",
+      "version": "3.2.3",
+      "resolved": "https://registry.npmjs.org/jws/-/jws-3.2.3.tgz",
+      "integrity": "sha512-byiJ0FLRdLdSVSReO/U4E7RoEyOCKnEnEPMjq3HxWtvzLsV08/i5RQKsFVNkCldrCaPr2vDNAOMsfs8T/Hze7g==",
       "license": "MIT",
       "dependencies": {
-        "jwa": "^1.4.1",
+        "jwa": "^1.4.2",
         "safe-buffer": "^5.0.1"
       }
     },
@@ -9694,12 +9670,12 @@
       "license": "MIT"
     },
     "node_modules/qs": {
-      "version": "6.13.0",
-      "resolved": "https://registry.npmjs.org/qs/-/qs-6.13.0.tgz",
-      "integrity": "sha512-+38qI9SOr8tfZ4QmJNplMUxqjbe7LKvvZgWdExBOmd+egZTtjLB67Gu0HRX3u/XOq7UU2Nx6nsjvS16Z9uwfpg==",
+      "version": "6.14.1",
+      "resolved": "https://registry.npmjs.org/qs/-/qs-6.14.1.tgz",
+      "integrity": "sha512-4EK3+xJl8Ts67nLYNwqw/dsFVnCf+qR7RgXSK9jEEm9unao3njwMDdmsdvoKBKHzxd7tCYz5e5M+SnMjdtXGQQ==",
       "license": "BSD-3-Clause",
       "dependencies": {
-        "side-channel": "^1.0.6"
+        "side-channel": "^1.1.0"
       },
       "engines": {
         "node": ">=0.6"